1. Field of the Invention
The present invention relates to a straddle-type vehicle, such as a motorcycle. More particularly, the present invention relates to an engine control system for a vehicle.
2. Description of the Related Art
Straddle-type vehicles, such as motorcycles, often adopt a constant-mesh transmission. Such transmissions include gears provided on each of a main shaft and a drive shaft. A gear change is effected by selectively changing the combination of these gears through the operation of a shifter device, such as a shifter arm or change pedal (“shifter”).
In a constant-mesh transmission, a driving force or output from the engine is transmitted via a clutch to the main shaft to be exerted on the drive shaft. Therefore, when a gear is to be changed, normally, the clutch is temporarily disengaged to release the driving force exerted on the drive shaft before performing a gear change.
However, some shift changes occur without a corresponding disengagement of the clutch. That is, under a state in which the accelerator position is significantly open and the engine driving force is large, accelerator release and opening operations are performed in quick succession (i.e., “snapping” or “blipping” is performed). This allows a gear change to be performed while the engine driving force is reduced for only a short period of time. The aforementioned operation is often referred to as “snapping.”
A technique for effecting the above-mentioned shift change operation omitting disengagement of the clutch is disclosed in, for example, Japanese Patent Publication No. 03-081537. The straddle-type vehicle disclosed in the publication includes a throttle opening sensor that detects the opening of a throttle valve. Ignition retardation processing is performed at the timing at which a change in the opening of the throttle valve due to movement of an operator-activated accelerator is detected by the sensor, thereby reducing the engine driving force. Accordingly, the engine driving force is reduced by the ignition retardation processing, thereby making it possible to reduce the operating force required for operating the shifter.